Shock absorbers are used in conjunction with automobile suspension systems and other vehicle suspension systems to absorb unwanted vibrations which occur during operation of the vehicle. To absorb this unwanted vibrations, shock absorbers are connected between the sprung mass (the body) and the unsprung mass (the suspension system) of the vehicle. A monotube shock absorber has a piston which is located within a pressure tube of the shock absorber and the piston is typically connected to the sprung mass of the vehicle using a piston rod. The pressure tube is typically connected to the unsprung mass of the vehicle and it is typically filled with hydraulic fluid. The piston includes valving systems which have the capability to limit the flow of hydraulic fluid within the pressure tube when the shock absorber is compressed (compression stroke) or extended (rebound stroke). The limiting of fluid flow produces a damping force which counteracts the vibrations which would otherwise be transmitted from the suspension (unsprung mass) to the body (sprung mass) of the vehicle.
A dual tube shock absorber comprises a pressure tube with a piston disposed therein and a reserve tube surrounding the pressure tube. The piston divides the pressure tube into an upper and lower working chamber. A piston rod is connected to the piston and the piston rod extends through the upper working chamber of the pressure and through the upper end of the reserve tubes. At the lower end of the pressure tube, a base valve is located between the pressure tube and the reserve tube. The base valve controls fluid flow between the working chamber defined by the pressure tube and a reserve chamber defined by the serve tube. Due to the piston rod being located on only one side of the piston within the upper working chamber, a different amount of fluid is displaced between the upper working chamber above the piston and the lower working chamber below the piston when the shock absorber extends or compresses. This difference in the amount of fluid is termed the “rod volume”. During a compression stroke, the “rod volume” flows out of the lower working chamber through the base valve and into the reserve chamber. During a rebound or extension stroke, the “rod volume” flows out of the reserve chamber through the base valve and into the lower working chamber. The piston rod is typically connected to the unsprung mass of the vehicle and the reserve tube is typically secured to the unsprung mass of the vehicle. During an extension or rebound stroke, a valving system in the piston limits the flow of hydraulic fluid within the pressure tube to produce a damping force. A check valve is included in the base valve to accommodate the “rod volume” flow of fluid. During a compression stroke, a valving system in the base valve limits the flow of hydraulic fluid between the lower working chamber and the reserve chamber to produce a damping force. A check valve is included in the piston to allow fluid to flow into the upper working chamber.
The piston rod of a shock absorber is supported at its lower end by the piston and it is slidingly received at the upper end of the pressure tube, and the reserve tube for a dual tube shock absorber, by a rod guide. The rod guide thus functions as a slide bearing for the piston rod. The rod guide properly positions the piston rod within the pressure tube and also acts as a closure member for both the pressure tube and the reserve tube when present. In order for the smooth sliding of the piston rod through the rod guide, a slight clearance is formed between the inner periphery of the bearing portion of the rod guide and the outer surface of the piston rod. This slight clearance allows for the hydraulic fluid to lubricate the interface between the piston rod and the rod guide.
In addition to locating the piston rod and closing the pressure tube and the reserve tube when present, the rod guide supports and locates a seal assembly which is designed to keep the hydraulic fluid within the shock absorber and also keep contaminants out of the shock absorber. The seal assembly normally interfaces between the rod guide and the piston rod and its purpose is to seal this interface in both rebound and compression strokes.
The prior art seal assemblies function well during a rebound stroke but they can be susceptible to leakage during a compression stroke. When the seal assembly is at zero velocity during the transition between rebound and compression, there is no residual force acting on the seal assembly. When the piston rod starts displacing in compression, the only initial force acting on the seal assembly is friction. This friction pulls the seal assembly down away from the rod guide until it meets the bearing bushing assembled with the rod guide. Depending on how well the bearing bushing is assembled, the dimensions of the bearing bushing and other related dimensions, the seal assembly then seals on the bearing bushing. The effectiveness of the seal is determined by how well the seal assembly and the bearing bushing are manufactured and the acting pressure (velocity) on the seal assembly. When the piston then displaces in rebound, the seal assembly will again be pushed upward against the sealing surface of the rod guide. This movement of the seal assembly between the rod guide and the bearing bushing provides a slight leaking of the hydraulic fluid. Typically, this leak is uncontrollable.
In addition to the problems associated with seal assembly movement, the problems associated with higher temperatures can cause leaking of the seal assembly. In applications which use the friction properties of the seal assembly for sealing, when higher temperatures are encountered, the seal assembly is again susceptible to leakage.